Intracellular bacterial pathogens engage in a tug-of-war with innate host defenses. In this issue of Cell Host & Microbe, Mostowy et al. (2010) identify a role for the septin family of cytoskeletal proteins in targeting intracellular Shigella to the autophagy pathway

Goldberg, Marcia B.

Heller, Danielle M.

Stamm, Luisa M.

Elsevier - Publisher Connector 

Cell Host & Microbe
PreviewsNK-cell function as a whole during the
chronic phase of infection. Clearly, the
study by Shah et al. (2010) marks only
the beginning of a fascinating story that
will shed new light on an important but still
poorly understood aspect of the interac-
tion of HIV-1 with NK cells.
REFERENCES
Alter, G., and Altfeld, M. (2006). Curr. Mol. Med. 6,
621–629.Bonaparte, M.I., and Barker, E. (2003). AIDS 17,
487–494.
Bryceson, Y.T., March, M.E., Ljunggren, H.G., and
Long, E.O. (2006). Immunol. Rev. 214, 73–91.
Cohen, G.B., Gandhi, R.T., Davis, D.M., Mandel-
boim, O., Chen, B.K., Strominger, J.L., and
Baltimore, D. (1999). Immunity 10, 661–671.
Kirchhoff, F. (2010). Cell Host Microbe 8, 55–67.
Lanier, L.L. (2008). Nat. Rev. Immunol. 8,
259–268.Cell Host & Microbe 8, NMoll, M., Andersson, S.K., Smed-Sorensen, A.,
and Sandberg, J.K. (2010). Blood 116, 1876–1884.
Richard, J., Sindhu, S., Pham, T.N., Belzile, J.P.,
and Cohen, E.A. (2010). Blood 115, 1354–1363.
Shah, A.H., Sowrirajan, B., Davis, Z.B., Ward, J.P.,
Campbell, E.M., Planelles, V., and Barker, E.
(2010). Cell Host Microbe 8, this issue, 397–409.
Ward, J., Davis, Z., DeHart, J., Zimmerman, E.,
Bosque, A., Brunetta, E., Mavilio, D., Planelles,
V., and Barker, E. (2009). PLoS Pathog. 5,
e1000613. 10.1371/journal.ppat.1000613.Caging Targets for DestructionLuisa M. Stamm,1,2 Danielle M. Heller,2 and Marcia B. Goldberg1,2,*
1Department of Medicine, Division of Infectious Diseases, Massachusetts General Hospital, Cambridge, MA 02139, USA
2Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115, USA
*Correspondence: mgoldberg1@partners.org
DOI 10.1016/j.chom.2010.10.010
Intracellular bacterial pathogens engage in a tug-of-war with innate host defenses. In this issue ofCell Host &
Microbe, Mostowy et al. (2010) identify a role for the septin family of cytoskeletal proteins in targeting intra-
cellular Shigella to the autophagy pathway.Several pathogenic bacteria, including
Listeria monocytogenes, Salmonella
spp., Legionella pneumophila, Yersinia
spp., and Shigella spp., cause disease
by entering into and surviving within cells.
These intracellular bacteria engage in
a tug-of-warwith host cell defenses, being
pulled between bacterial restriction and
death on one side and dissemination
and survival on the other. In the time
immediately after infection, the former is
mediated to a large extent by the host
cell autophagy pathway. In this pathway,
intracellular bacteria are engulfed in intra-
cytosolic membrane-bound vacuoles
called autophagosomes that fuse with
lysosomes, leading to bacterial destruc-
tion (Deretic and Levine, 2009). Shigella
spp. are among a subset of intracellular
bacterial pathogens that, after entry into
cells, escape the vacuole and replicate in
the cytosol. Once in the cytosol, each
member of this group, which includes
Shigella spp., L. monocytogenes, Bur-
kholderia spp., Rickettsia spp., and
Mycobacterium marinum, induces actin
polymerization on its surface, whichpropels it to the cell periphery, whereupon
it spreads into adjacent cells (Stevens
et al., 2006). The ability of these organisms
to spread directly into adjacent cells is
critical to virulence. Of the population
of organisms that enter the cytosol in a
given infection, a substantial percentage
escape autophagy, while others are
brought into the autophagy pathway
where they are destroyed (Ogawa et al.,
2005). The intracellular fate of these cyto-
solic bacteria is a delicate balance
between bacterial and host factors. For
Shigella, autophagy proteins bind the
bacterial protein IcsA (VirG), which is
required for actin-based motility, and this
interaction is inhibited by a second bacte-
rial protein, IcsB (Ogawa et al., 2005). Few
host factors that determine the fate of
intracellular Shigella have been previously
identified. In this issue, Mostowy et al.
(2010) demonstrate a role for the host
septin family of cytoskeletal proteins in
targeting intracytosolic Shigella flexneri
to the autophagy pathway.
Septins are a large class of proteins
involved in many diverse cellular pro-cesses that have been shown to function-
ally interact with cellular membranes and
the cytoskeleton. Originally discovered in
1971 through a genetic screen for yeast
mutants defective in budding, septins
assemble into rings at the yeast bud
neck and are thought to act as membrane
diffusion barriers that separate themother
from the daughter cell (Cao et al., 2009).
Septins are conserved across animals
and fungi, but are absent in plants. Hu-
mans have 14 septins that are divided
into four groups, among which there is
partial functional redundancy. Impor-
tantly, septins from the different groups
form hetero-oligomeric protein com-
plexes that assemble into nonpolar fila-
ments (Weirich et al., 2008). The variation
in septin proteins and their ability to form
higher order hetero-oligomeric structures
translates into many combinatorial possi-
bilities, leading to distribution throughout
the cell and multiple diverse protein inter-
actions and functions.
Several notable functions of septins
relate to their ability to interact with
membranes. In addition to serving asovember 18, 2010 ª2010 Elsevier Inc. 391
Figure 1. Model of the Tug-of-War between Intracytosolic Shigella and Host Cell Defenses
Early after Cellular Infection, as Demonstrated by Mostowy et al. in the Current Issue of Cell
Host & Microbe
A subset of intracytosolic Shigella polymerizes actin tails and spreads into adjacent cells, while other intra-
cytosolic Shigella are trapped in cages formed by cellular septin proteins. Septin cages play a critical role
in targeting Shigella to the autophagocytic pathway. Myosin II contributes to the targeting of Shigella to the
autophagosome, and its inhibition enables Shigella to escape from septin cages.
Cell Host & Microbe
Previewsmembrane diffusion barriers in yeast, in
mammalian cells, septins interact with
the plasma membrane via a conserved
polybasic region in the N terminus that
binds membrane-associated phosphati-
dylinositol (4,5)-bisphosphate and phos-
phatidylinositol (3,4,5)-triphosphate (Cao
et al., 2009). Moreover, the mammalian
septin SEPT5 plays a role in the fusion
of exocytic vesicles with the plasma
membrane through its direct association
with SNARE proteins (Cao et al., 2009).
The intrinsic bent shape of septin fila-
ments seen in crystal structures may
allow for it to accommodate interactions
with the curvature of membranes.
While able to connect with cellular
membranes on one hand, septins interact
with both the actin and the microtubule
cytoskeleton on the other (Spiliotis,
2010). Biochemical reconstitution studies
show that septins polymerize into fila-
ments using F-actin fibers as a tem-
plate and actin-binding proteins such as392 Cell Host & Microbe 8, November 18, 201myosin II as adaptors. Six types of mam-
malian septins have been shown to coloc-
alize with microtubules, in a process that
may be facilitated by an association of
microtubules with the GTPase domain at
the N terminus of septins. The GTPase
binding domain and the membrane-
binding polybasic domain, which inter-
acts with membranes, form distinct inter-
faces in the crystal structure (Weirich
et al., 2008), suggesting that simulta-
neous interaction with membrane and
cytoskeleton may be possible. Given their
ability to interact with both membranes
and the cytoskeleton, septins are ideally
poised to function in cellular trafficking.
Indeed, SEPT2 and SEPT11 localize with
cortical actin at the plasma membrane
during Fc receptor phagocytosis (Cao
et al., 2009).
In the current issue of Cell Host &
Microbe, Mostowy et al. (2010) make
a compelling argument that septins play
a critical role in barricading the intracel-0 ª2010 Elsevier Inc.lular trafficking of Shigella, thereby inhibit-
ing dissemination and survival. Only
a subset of intracytosolic Shigella assem-
bles actin tails. Mostowy et al. demon-
strate that intracytosolic Shigella without
actin tails are surrounded by septin ‘‘ca-
ges’’; using immunofluorescence, they
show that the septins SEPT2, SEPT9,
and SEPT11 each form spirals along the
length of the bacterium. Interestingly,
these three septins belong to different
groups in the septin family and therefore,
in theory, may be associating into
a hetero-oligomeric complex. Using inter-
ference RNA to deplete SEPT2 or SEPT9,
the authors demonstrate a functional role
for each of these two septins, finding that
in their absence, septin cages are formed
significantly less frequently and the per-
centage of intracellular bacteria associ-
ated with actin tails is significantly
increased. SEPT11 depletion does not
inhibit colocalization of SEPT2 and
SEPT9, perhaps due to functional redun-
dancy that does not apply to SEPT2 and
SEPT9. The association of septin cages
predominantly withShigella that lack actin
tails is consistent with amodel in which in-
tracytosolic Shigella are either encaged
by septins in conjunction with other
cellular factors or are able to escape
host cell capture by assembly of an actin
tail (Figure 1).
Consistent with prior publications of an
association of septins with cytoskeletal
components (Cao et al., 2009; Spiliotis,
2010), Mostowy et al. (2010) show that
host cytoskeletal actin, N-WASP, andmy-
sosin II not only colocalize with but also
have functional roles in the formation of
septin cages around Shigella. The pres-
ence of actin polymerization and
N-WASP are required for the association
of septins with Shigella (Mostowy et al.,
2010), perhaps contributing to the previ-
ously reported requirement for IcsA in
the recruitment of autophagy markers
(Ogawa et al., 2005). Labeled actin or
N-WASP forms spirals that alternate with
those of septins, suggesting that each
one’s role in septin assembly is indirect.
In contrast, myosin II labeling overlaps
with septin, perhaps indicating that
myosin II plays a more direct role in septin
assembly or stabilization. The results are
consistent with the ability of septins to
use F-actin as a template for filament
formation with myosin II functioning as
an adaptor between the two. Using
Cell Host & Microbe
Previewspharmacologic inhibitors, Mostowy et al.
(2010) show that association of septin ca-
ges around Shigella is dependent upon
myosin II activity. Disruption of myosin II
activity enables bacteria to ‘‘break free’’
from the septin cage and initiate actin-
based motility, clearly demonstrating the
switch from one intracellular fate, that of
septin association, to another, that of
actin polymerization and intercellular
spread (Figure 1).
Most interesting among the findings
from Mostowy et al. (2010) is a previously
unrecognized link between septins and
the autophagy pathway in mammalian
cells. Many markers of the autophagy
pathway colocalize with Shigella in septin
cages, suggesting that the purpose of
the cagesmay be not only to prevent actin
polymerization, tail formation, and spread
of bacteria, but also to target intracyto-
solic bacteria for degradation. Mutant
Shigella lacking IcsB have been shown
to enter the autophagy pathway with
increased frequency (Ogawa et al.,
2005). Mostowy et al. (2010) found that
an icsB mutant is significantly more
frequently associated with septin cages,
while an icsA mutant, which has been
previously shown to be relatively resistant
to autophagy (Ogawa et al., 2005), is
significantly less frequently associated
with cages. These results seem to suggest
a codependence of septins with factors
that mediate autophagy; when septins
are absent, autophagy markers accumu-late less, and vice versa. Thus, these
results indicate an ongoing pull between
host and bacterial factors in determining
the fate of intracellular Shigella.
The dynamics of higher order septin
assembly and function is complex and
incompletely understood, yet is likely
coordinated by a variety of intracellular
and extracellular cues. The findings
presented in Mostowy et al. (2010)
suggest that one player in this process is
the proinflammatory cytokine TNF-a,
which is known to be important in host
responses to intracellular bacteria. Addi-
tion of TNF-a to cells prior to infection
with Shigella increases septin assembly
on intracytosolic bacteria and correlates
with less frequent actin tail formation
and bacterial dissemination. Thus, the
cytokine TNF-a modulates septin as-
sembly and targeting of cytosolic Shigella
to the autophagy pathway.
Taken together, the findings presented
in Mostowy et al. (2010) demonstrate
a continuous struggle between intracyto-
solic Shigella and the host autophagy
pathway, in which the assembly of septin
cages plays a critical role (Figure 1).
Precisely how these findings can or will
be generalized to other intracellular
bacteria is uncertain. The authors indicate
that while L. monocytogenes and Rick-
ettsia conorii do not associate with septin
cages, M. marinum, which is thought to
have amechanism of actin polymerization
more similar to that of Shigella, doesCell Host & Microbe 8, Nassociate with them. These data suggest
that certainmechanisms of actin polymer-
ization may be better suited for septin-
mediated entrapment than others.
Whether L. monocytogenes andR. conorii
might be captured by a parallel septin-
independent pathway and whether other
intracellular bacterial pathogens are
captured by a septin-dependent mecha-
nism remain unknown. Nevertheless, this
paper contributes to our insight into the
mechanisms involved in the tug-of-war
between host and intracellular pathogen,
identifying septin proteins as key players
in this struggle.REFERENCES
Cao, L., Yu,W., Wu, Y., and Yu, L. (2009). Cell. Mol.
Life Sci. 66, 3309–3323.
Deretic, V., and Levine, B. (2009). Cell Host
Microbe 5, 527–549.
Mostowy, S., Bonazzi, M., Hamon, M.A., Tham,
T.N., Mallet, A., Lelek, M., Gouin, E., Demangel,
C., Brosch, R., Zimmer, C., et al. (2010). Cell Host
Microbe 8, this issue, 433–444.
Ogawa, M., Yoshimori, T., Suzuki, T., Sagara, H.,
Mizushima, N., and Sasakawa, C. (2005). Science
307, 727–731.
Spiliotis, E.T. (2010). Cytoskeleton (Hoboken) 67,
339–345.
Stevens, J.M., Galyov, E.E., and Stevens, M.P.
(2006). Nat. Rev. Microbiol. 4, 91–101.
Weirich, C.S., Erzberger, J.P., and Barral, Y.
(2008). Nat. Rev. Mol. Cell Biol. 9, 478–489.ovember 18, 2010 ª2010 Elsevier Inc. 393


Caging Targets for Destruction 

file:///data/remote/core/dit/data/elsevier/pdf/cf5/aHR0cDovL2FwaS5lbHNldmllci5jb20vY29udGVudC9hcnRpY2xlL3BpaS9zMTkzMTMxMjgxMDAwMzQ3MQ==.pdf

Caging Targets for Destruction

Abstract

Similar works

Full text

Available Versions

Elsevier - Publisher Connector

Elsevier - Publisher Connector